Figure 15: Bragg peak position shift and scaling of different orders. Detailed analysis of the photo-induced changes of the Bragg peaks for the pattern shown in Figure 6. (a) the profiles of the 008 and 0010 Bragg spots are shown at selected time delays. The profiles for time delays between 0 and 112 ps are displaced vertically for clarity. The time delays for the curves displayed in the top panel are 233, 33, 4, 8, 20, 32, 44, 56, 68, 80, 92, and 112 ps, for the curves in the bottom panel 112, 147, 217, 317, 617, 917, and 1217 ps. For the curves in the top panel, a structural isosbestic point can be seen for both Bragg spots (see text). The curves in the bottom do not cross; the center positions of the Bragg spots shift back to equilibrium continuously. (b) the relative changes of the center positions of 0010 (blue) and 008 (red) Bragg spots and direct electron beam (black) are plotted as a function of time. The center positions are obtained by fitting the curves in (a) to a single Gaussian form. The fit to such function is good except for the region shown by a transparent yellow strip during which a single Gaussian cannot describe the profiles since at least two structural phases coexist. We also plot the shift of the 0010 Bragg spot (blue curve) scaled by %80 as the dashed blue curve. The agreement between this and the red curve is consistent with a true structural change in which the shifts of the Bragg peaks scale proportionally to the order numbers since . Furthermore, no substantial movement of the direct beam is observed, indicating that the observed effects cannot be coming from shifting of the entire pattern.